1. Field of the Invention
The invention relates to integrated-optics interferometers, and in particular to an interferometer fabricated on a single LiNbO.sub.3 chip which is useful for stabilization of broad-bandwidth optical sources.
2. Description of the Related Art
Stability of the operating characteristics of an optical source is often a necessity in the design of a fiberoptic sensing system. For example, highly accurate fiberoptic rotation sensors require a light source with a stable wavelength. The scale factor of a rotation sensor, defined as the ratio of the output signal to the input rotation rate, depends on the wavelength of the source, and variations in source wavelength give rise to measurement inaccuracies. For rotation sensors used in navigation, a fractional variation in source wavelength of no greater than one part in a million may be required.
One type of light source which has found use in fiberoptic sensing systems is the broadband semiconductor light source, as typified by the the superluminescent diode (SLD). The use of of an SLD reduces noise from the coherent backscattering that takes place in the fiber with semiconductor diode lasers. A superluminescent diode has a broad enough spectral linewidth to obviate undesirable phase errors caused by coherent backscattering and the Kerr effect. The wavelength of light emitted from a laser diode varies with the operating temperature of the diode and with the current injected into the diode. The temperature of the diode must be held as constant as possible, and the injection current must also be regulated to maintain a stable output wavelength. The fractional shift in the centroid of the spectral output distribution must be of the order of 1 part in 10.sup.6 to satisfy the linearity and scale-factor stability requirements of high-grade fiberoptic rotation sensors.
Developments in the field of optical communications have led to the availability of high-quality, low-loss single-mode optical fibers and fiber components which have found wide-ranging applications in sensing technology. Single-mode fiber sensors are usually used in forming interferometers to take advantage of the techniques of optical interferometric sensing. Optical interferometers provide an unrivalled sensitivity in the detection of displacements down to the order of a thousandth of an Angstrom unit, which is equivalent to one billionth of the diameter of a human hair. Before the advent of optical fibers, conventional interferometers using bulk optical components such as beamsplitters and mirrors suffered from unwanted sensitivity to environmentally induced misalignments. The fabrication of interferometers from single-mode optical fibers eliminates this problem because the two light beams are guided along an optical path formed by the fibers themselves.
The commonest types for two-beam fiber interferometers are the Michelson interferometer and the Mach-Zehnder interferometer. The Michelson configuration is the simpler of the two, with the light from an optical source of wavelength X being divided in amplitude by a beamsplitter/directional coupler to give a reference beam and a signal beam which propagate down fiber stubs with reflecting ends. The reflected beams return to the beamsplitter/coupler where they are coherently recombined into an output signal which is detected by a photodetector.
Any difference between the optical path lengths of the in the fiber arms of the interferometer results in a change in the output signal, with a path difference of X/2 producing a shift of one interference fringe. The output of the photodetector as a function of path difference is proportional to (1+cos .phi.), where .phi. is the phase difference between the two light beams caused by the difference in optical path length between the two arms of the interferometer.
The Mach-Zehnder configuration is slightly more complicated than the Michelson, requiring an additional beamsplitter/coupler. It offers two advantages over the Michelson configuration. First, optical feedback to the light source is reduced, which is important when semiconductor laser sources are employed. Second, there are two outputs from the second coupler which are 180 degrees out of phase with each other, of the form (1+cos .phi.) and (1-cos .phi.). The two outputs are equal only at the quadrature point, and can be conveniently used as the inputs to signal processing electronics to keep the interferometer at maximum sensitivity.
Devices for stabilization of the wavelength of the optical source which have been previously proposed have been either bulk-optic devices or fiberoptic devices. This is particularly true for stabilizing single-frequency lasers. The stabilization is accomplished by either locking the source wavelength to the absorption line of an element such as rubidium or iodine or by locking to a stabilized Fabry-Perot or Michelson interferometer. These methods, however, which are successful in stabilizing narrow-line sources with long coherence lengths, are ineffective in stabilizing broad-bandwidth sources with short coherence lengths.
Nav-grade performance of the fiberoptic gyroscope (FOG) requires that the wavelength of the optical source be stabilized. Because the preferred optical sources for FOGs are superluminescent laser diodes (SLDs), short-optical-pathlength-difference polarimetric interferometers have been used to stabilize the source wavelength. These are bulk-optic devices which necessitate strict constraints on the optical thicknesses of the two thermally compensated crystals which are the crux of the devices. In addition, the performance of these bulk-optic devices is sensitive to temperature differences between the crystals.
Because it is fabricated on a single chip, both the optical pathlength difference (opd) and the temperature gradients of an integrated-optics wavelength stabilization unit (IOWSU) can be carefully controlled. In addition, the integrated-optics technology of fabricating these devices is well developed, and these devices can be produced readily in large quantities.